Cable sizes , just in case.

[richardrich79]

The consumer unit in my house is just inside the front door and the majority of cables run up through the ceiling & under the 1st floor.
We are in the process of laying a new bathroom floor (1st floor, no electrics in B/Room) and part of our ongoing plans are to extend the kitchen and a disabled facilities shower-room.
We already have an electric shower and cooker but as there are 2 available spaces empty in the c/u I am considering future proofing by laying new cables under the floor to cover any upgrade needed should the existing shower and cooker cables not be the correct spec.
The electrician will not be around until the kitchen/shower part of the re-furb' starts. I shall not be connecting anything, just laying the cable.
The electric shower will be 10.5kw max and I don't know what cooker yet but want to cover all eventualities.
What size cables should I use.

 

[riveralt]

10mm ² T&E - should cover most eventualities.
However, you will probably also need to upgrade your MCB and the circuit will have to be additionally protected by an RCD.
If you intend to run two electric showers you will have to inform your DNO.

 

[ban-all-sheds]

The electrician will not be around until the kitchen/shower part of the re-furb' starts. I shall not be connecting anything, just laying the cable.

Don't be at all surprised if you find electricians reluctant to use that cable.

You'll be asking them to sign documents to say that their new circuits comply with the Wiring Regulations and the Building Regulations, and yet they will have no idea exactly where you'll have put the cable, how you protected it if necessary etc.

 

[richardrich79]

I should have mentioned that the only area that the cable(s) will be hidden is a 1.75m run under the new bathroom floor. He/she will see both where it goes under the floor and where it emerges from the ceiling below. The cables will still be 'on the roll' as I intend to let the sparks do whatever needs doing regarding clips etc on the rest of the runs. The rest of the floorboards can be lifted as required it's only the small area in the bathroom where the floor is being replaced then tiled which wont be accessible.
One guy has told me 10mm will be ok for the shower and 6mm for the cooker (oven only). The existing shower is 6mm which could be extended.
Thing is I am disabled and on a tight budget and thankfully some people being sympathetic & trying to help us find ways to make savings. I am very very grateful but I want the job done only once as if it need doing again in the future I doubt I'll be able to afford it.

 

[ericmark]

ban-all-sheds is correct so what you need to do is have it connected to something so it has a certificate raised for example a cooker isolator or second consumer unit.

I have had problems in the past energising cables which I thought I knew the route of only to find there was a junction box I did not know about which resulted in another cable becoming live.

I have also had cable damage and in spite of doing an insulation resistance test I did not find the damage and a nail became live.

I have also found where a cable went through heavy insulation so it's current rating was well below what I at first thought.

As a result I would not any longer use a cable which was not in use or unless paperwork existed change a protective device for a larger one unless I could see the cables route.

To fit a new cable into a consumer unit and fit a new MCB/RCBO is making a new circuit and as such we need to make out an insulation certificate however taking a cable from for example a cooker isolator and routing to a shower is not a new circuit so we only need to complete a minor works certificate. With a minor works we can show a limitation and it would then not cover the original part of the installation.

Having said that I have also installed some cables ready in my mothers house but the big difference is I will be the one using them.

 

[SaladFingersMKII]

I have also had cable damage and in spite of doing an insulation resistance test I did not find the damage and a nail became live.

Well, an insulation resistance test is only to test dielectric strength, it's not there to tell you anything else.

 

[richardrich79]

I that case I guess all I can do is take photos as I go and let the sparks connect up the existing smaller cables simply by extending them as they have already said they'll do.
If in the future there's a problem I'll just have to diy using the 10mm unused cables

 

[JohnW2]

Don't be at all surprised if you find electricians reluctant to use that cable. ... You'll be asking them to sign documents to say that their new circuits comply with the Wiring Regulations and the Building Regulations, and yet they will have no idea exactly where you'll have put the cable, how you protected it if necessary etc.

Whilst that is all true, it could be said that there is, at least in terms of safety (which is what it's meant to all be about) an inherent silliness in it. The OP only wants to put in these new cables in case the existing cables prove to be unsatisfactory. If the existing cables appear satisfactory (correct CSA and test OK) then one imagines that most electricians would happily use them, without any knowledge of the routing/protection of the cable - and the OP's new cable could not be any more 'unknown' than that, particularly if he took photos of the bits that were going to be 'concealed' from the electrician.

Ironically, if the OP actually connected his new cables, the potential problem would probably then go away. At least in England, he could replace the shower and cooker cables without the need for notification. The electrician would then be presented with 'existing cables' which, as above, he would presumably be happy to continue using if they were satisfactory (adequate CSA and tested OK), as above, without having to 'sign for' their routing/protection etc. Maybe a sensible electrician would allow the OP to go through the motions of this 'connecting' of the new ('replacement') cables under supervision, thereby rendering them 'existing cables' as far as the electrician was concerned (or even just 'imagine' that they had been through that process)?

Kind Regards, John

 

[mfarrow]

Just put in 2x 25mm plastic conduit with draw string in the bathroom, with a long radius bend down to the CU. Take some photos as a precaution.

Conduit's cheap compared to cable you might never use.

 

[richardrich79]

Just put in 2x 25mm plastic conduit with draw string in the bathroom, with a long radius bend down to the CU. Take some photos as a precaution.

Conduit's cheap compared to cable you might never use.

Thanks for the idea but I'm not too sure the current 6mm cable to our present shower will be ok if I stick a 9 or 10kw shower in its place (the wire extended by 5m to the ground floor)
However I will stick a bit of conduit or larger plastic pipe in complete with bits of string,Brilliant thanks

 

[mfarrow]

Yes your 6mm cable will struggle with compliance for a shower of the size you want to fit.

Electricians are a lot more comfortable with drawing new cables into existing containment than using others' cables. Proper conduit will be more assuring to them than pipe; seeing SpeedFit's finest might remind them of the T&E they'll have seen running in hosepipe 2" under to a customer's shed.

Wickes is the cheapest shed for these sorts of bits in small quantities, I find.

 

[Satori]

[...] our ongoing plans [involve] a disabled facilities shower-room.

[...] I am disabled and on a tight budget and thankfully some people being sympathetic & trying to help us find ways to make savings.

The Building (Local Authority Charges) Regulations 2010, regulation 4, provides a fees exemption to work(s) carried out "for the purpose of [...] providing facilities designed to secure the greater health, safety, welfare or convenience of the disabled person."

If this regulation is applicable, certification can be sought from your local building control body for the installation of the new circuit.

A 10.5 kW (rated assuming 240 V) shower should not be supplied by 10 mm² T&E if the cable is routed through conduit or trunking. 10 mm² PVC insulated single cores are fine in conduit/trunking however if using 10 mm² T&E it should be clipped direct.

You could install a second 6 mm² T&E cable along the original route to the shower to provide parallel conductors. This will be more economical than installing a new 10 mm² cable. If you must use conduit/trunking, use either two pairs of 6 mm² PVC insulated single core conductors (or two T&E 6 mm²) in parallel, or one pair of PVC insulated single cores at 10 mm². The conduit itself should conform to IEC/BSEN 61386.

Regulation 4 mentioned above: http://www.legislation.gov.uk/uksi/2010/404/regulation/4/made

National Assistance Act 1948, sec. 29(1) [ignore any reference to age in this subsection]: http://www.legislation.gov.uk/ukpga/Geo6/11-12/29/section/29[/i][/u]

 

[mfarrow]

A 10.5 kW (rated assuming 240 V) shower should not be supplied by 10 mm² T&E if the cable is routed through conduit or trunking. 10 mm² PVC insulated single cores are fine in conduit/trunking however if using 10 mm² T&E it should be clipped direct.

Even considering the worst case (T&E in conduit in insulating wall) 10mm² will be fine for a 10.5kW shower, which will pull 43.75A, lookin at Table 4D5.

 

[Satori]

The OP in complying with R433.1.1(ii), which he is not bound by law to observe, ought to choose in Table 4D5A of BS 7671 that ampacity, subject to any necessary derating, which at least equals the magnitude of the OPD rating.

In his case the OPD rating will be 45 A or greater. The tabulated ampacity for 10 mm² T&E, method A, is 44 A. In the absence of further derating he could use 10 mm² on the basis of there being a lower thermal resistance associated with conduit attached to a surface than one buried in an insulated wall. Most of us, I suspect, would be content to accept that fact without further consideration and I imagine many experienced electricians here view my comments on this point as being unduly pedantic.

The point bears some resemblance to a question I raised on this forum recently. I am also installing a shower although in my case I am forced to consult IEC 60287 to prove that my distribution conductors will not exceed 70 centigrade (since according to table 4D5A they will be overloaded). The OP will likely have little difficulty convincing a LABC inspector that 10 mm² T&E installed by method B, a method not considered by the aforementioned table, is deemed to satisfy R433.1.1(ii) [Iz >= In]. Having said all of that, the easiest solution is to assume an ambient temperature of 26 centigrade.

 

[richardrich79]

This is the 'pinch-point' I'm trying to avoid any pore sparks having to try and feed a cable through once the floor is laid.

[/url]

Untitled

• richardrich79
• 4 Oct 2013
• 1

I do intend to, very carefully enlarge the notch cut-out and ease the congestion.